The present invention pertains to the field of steady state (DC) pressure and temperature measurement. More particularly, the present invention pertains to the field of sensors for measuring steady state pressure and temperature of a moving fluid, such as is used in drilling for oil and natural gas, and especially to sensors using optical fibers and having a Bragg grating.
Because of various advantages including for example immunity to electromagnetic interference and especially reliability in a high temperature environment, it is desirable to measure the static or so-called DC pressure and temperature of a moving fluid using sensors based on optical methods. In addition, though, such a sensor should be robust. In case of measuring the pressure and temperature of a moving fluid in a harsh environment, it is advantageous to provide a sensor that will yield precise and accurate measurements over a long period of time, i.e. to provide a sensor whose operating characteristics will not significantly change over a useful lifetime of the sensor even in the harsh environment. One application where a robust DC pressure and temperature sensor is needed is downhole in extracting oil and gas from a drilled site. There it is the DC pressure and temperature of the production fluid (mixtures of oil, gas and water) that is sought to be measured.
In some applications of sensing parameters of a process, optical fibers are wrapped around a mandrel that deforms in response to changes in the process parameter being sensed and measured. To make a robust sensor using optical fibers wrapped around a mandrel that deforms in response to changes in DC pressure or temperature, it is important to keep the optical fibers from being under too high a constant tension, which would cause the optical fibers to undergo creep (involving a failure of the bonding agent adhering the optical fibers to the mandrel) and would also shorten the life of the optical fibers. The prior art does not teach how to keep tension low in optical fibers being used in a mandrel-based DC pressure and temperature sensor, and thus to make a robust mandrel-based sensor.
Thus, what is needed is a robust, optical-fiber-wound, mandrel-based sensor for sensing information about DC pressure and temperature of a moving (flowing) fluid, and in particular for sensing information about DC pressure and temperature of a production fluid in oil and gas extraction from a drilled site.
Accordingly, the present invention provides a DC pressure and temperature sensor system, for sensing and measuring the DC pressure and temperature of a production fluid in tubing. It includes at least one fluid sensor, but sometimes two. Only one is needed if either the DC pressure or temperature of the production fluid (but not both) is provided by an independent measurement. In case that no such independent information is available, the DC pressure and temperature system includes: a first fluid sensor and a second fluid sensor, the first fluid sensor using a first sensing material (preferably one or another type of oil) and the second fluid sensor using a second sensing material in which sound travels at a rate that depends on the DC pressure and temperature of the second sensing material in a measurably different way than for the first sensing material. Each sensing material is coupled to the production fluid, preferably via a thin-walled membrane, so as to be at a DC pressure and temperature that can at least be correlated to the DC pressure and temperature of the production fluid and is preferably the same as the DC pressure and temperature of the production fluid. Each fluid sensor is responsive to the DC pressure and temperature of the production fluid via the coupling of the production fluid to the sensing materials, and via monitoring the speed of sound in the sensing materials. Each fluid sensor provides sensing signals containing information about the DC pressure and temperature of the production fluid. The information relates directly to the speed of sound in the sensing material of each fluid sensor, and the information from each fluid sensor (two pieces of information) is used to determine the two quantities to be measured: the DC pressure and temperature of the production fluid.
In a further aspect of the invention, at least one of the fluid sensors includes: a mandrel, enclosing the respective sensing material, responsive to changes in DC and acoustical pressure and temperature of the corresponding sensing material, for providing a change in diameter corresponding to changes in DC and acoustical pressure and temperature of the corresponding sensing material; and an array of sensing means affixed to the mandrel, responsive to changes in the diameter of the mandrel, for providing the signals containing information about the DC pressure and temperature of the production fluid.
Essentially, each sensing means in the array of sensing means of a fluid sensor is used to detect the propagation of acoustic waves. Since an array of such sensing means is used, the speed of the acoustic wave (i.e. the speed of sound) in the sensing material can be determined. The speed of sound in a material depends only on the DC pressure and temperature of the material, and so each fluid sensor, by providing a measure of the speed of sound in the sensing material used by the fluid sensor, provides one of two pieces of information needed to determine the two quantities, DC pressure and temperature of the sensing material, the two quantities being the same or correlated (because of coupling through the membrane) as for the production fluid.
In a still further aspect of the invention, at least one sensing means is a winding of an optical fiber about the mandrel, the winding having an associated Bragg grating, and the winding is responsive to a narrowband light signal.